The present invention relates to a magnetic recording medium which is used in a magnetic recording apparatus such as a hard disk drive and more particularly to a perpendicular magnetic recording medium which is used in a recording system which records magnetization almost perpendicular to a substrate surface.
Currently the mainstream in recording systems for use in hard disk drives is a longitudinal recording system in which magnetization is recorded along the substrate in-plane direction. However, in order to realize a higher recording density, or a larger capacity hard disk drive, research has been actively conducted on perpendicular recording in which magnetization is oriented perpendicularly to the substrate. A recording medium which is used for perpendicular magnetic recording has a magnetic easy axis oriented almost perpendicularly to the substrate and consists of a magnetic recording layer for holding record, a soft magnetic layer for efficient use of the magnetic field of a magnetic head and so on. In perpendicular recording, since magnetization in boundaries (magnetization transition regions) of recorded magnetization regions is anti-parallel, magnetic stability is ensured and the width of the so-called “zigzag magnetic domain wall” is small and therefore medium noise is reduced. The soft magnetic layer not only functions as a return path for the magnetic head's magnetic field; but when it is used with a single pole head, it generates a strong magnetic field in the magnetic recording layer due to a mirror effect, which makes it possible to use a recording magnetic film with a high coercivity and thus improves thermal stability in recorded magnetization.
It has been thought that as a condition for a fine structure of medium for reduction in medium noise of a perpendicular recording medium, the grain size of crystal grains must be very small and exchange interaction between neighboring crystal grains must be small. The reason for this is that because a unit of magnetization reversal is one crystal grain as a constituent of a magnetic recording layer or a combination of plural crystal grains, the width of the magnetization transition region largely depends on the size of this unit of magnetization reversal. However, there was a problem that an extremely small grain size might lower thermal stability of grains and even a thermal energy equivalent to room temperature might cause reversal of magnetization, namely demagnetization, with a high probability. Hence, regarding conventional perpendicular magnetic recording media, research and development has been pursued to increase the magnetic distance between grains while holding grain boundaries non-magnetic under the policy that intergrain interaction should be reduced while the grain size is held at about 10 μm. In order to achieve this, a so-called “granular” structure magnetic recording medium in which the peripheries of magnetic crystal grains (crystal grain boundaries) are surrounded by non-magnetic layers for the purpose of reducing interaction between crystal grains has been proposed and developed. For example, JP-A No. 358615/2002 discloses a granular magnetic recording medium with an average intergrain distance of 1.0 nm or more. This disclosure document describes that the medium uses an oxide, nitride, fluoride or carbide for a grain boundary layer. Also, JP-A No. 178413/2003 discloses a granular magnetic recording medium which uses an oxide for a non-magnetic layer which constitutes a grain boundary layer where the volume of oxide contained in a magnetic recording layer is specified to ensure a high coercivity. For comparison, when a magnetic recording medium used in a longitudinal recording system uses CoCrPt alloy for its magnetic recording layer, a granular structure can be made without addition of an oxide or the like and intergrain interaction can be reduced in a relatively adequate manner because a non-magnetic element such as Cr can be segregated in grain boundaries. On the other hand, when only CoCrPt alloy is used for the magnetic recording layer of a perpendicular recording medium, segregation of Cr in grain boundaries hardly occurs and it is difficult to make a sufficiently segregated grain boundary structure, or a granular structure. Therefore, in perpendicular recording media, grain boundary formation has been stimulated by addition of an oxide to make a granular structure for noise reduction.